In a conventionally known high density information recording and reproducing system, information is reproduced from a recording medium such as an optical disk or an optical card by projecting a converging light beam upon the recording surface of the optical recording medium and detecting a light beam reflected therefrom with a light measuring sensor such as a photodiode device. The information is written in a large number of pits formed along a recording track defined in the recording medium. Recording of information in the optical recording medium is carried out by projecting a relatively more powerful light beam such as a laser beam upon such pits and selectively altering their reflective properties according to the information to be recorded.
Since the optical recording medium for such a system generally involves warping and distortions, and may have an eccentricity due to positional errors in the mounting of the optical recording medium, it is necessary to control the position of the optical head so that the light beam from the optical head may accurately follow the recording track and a properly focused condition may be maintained at all times Therefore, it is necessary to detect the focusing and tracking errors of the optical head so that the optical head may be appropriately controlled according to such detected tracking and focusing errors.
There are a number of known structures for detecting focusing and tracking errors. For instance, there is known the three beam method in which a diffraction grating is placed in an optical path of a light beam emitted from a semiconductor laser serving as a light source to produce a 0th-order diffraction light beam and .+-.1st-order diffraction light beams so that the main light beam reflected from the optical recording medium and consisting of the 0th-order diffraction light beam may be used for detecting focusing errors as well as for reading and writing information and the reflected sub light beams consisting of the .+-.1st-order diffraction light beams may be used for detecting tracking errors. The tracking error can be detected as the difference between the magnitudes of the sub beams reflected from the recording surface of the recording medium.
Conventionally, the detection of focusing errors in such a three beam method was typically based on the astigmatism method, in which the light spot obtained from the main beam reflected from the recording surface becomes elongated in either one of two mutually perpendicular directions depending on the direction of the focusing error by means of the use of a semi-cylindrical lens in the optical system, and a proper focusing is indicated by the light spot being substantially circular. The light measuring sensor for such focusing error detection consists of a four-segment photodiode, and the focusing error is detected by computing the difference between the outputs from the pairs of the diagonally opposed segments of the photodiode.
However, according to such a known structure, when a tracking error is produced, due to the shift of the main beam in the direction of the tracking error, the focusing error signal tends to be strongly affected by the tracking error. When such cross talk develops between the focusing error signal and the tracking error signal, the focusing control tends to be destabilized, and this leads to the destabilization of the tracking control. In extreme cases, the stable control of the optical head becomes impossible.
Also, the focusing error signal obtained from such a four-segment photodiode has a poor linearity, and an optimum focusing control is difficult to achieve.